Corrosion-resistant austenitic-ferritic stainless steel

ABSTRACT

A corrosion resisting steel with considerably higher yield point and ultimate tensile strength as well as good ductility and impact strength is achieved by adding a comparatively large quantity (2-4% by weight) of Si to a high chromium austeniticferritic steel. The resultant steel typically has a composition (percent by weight):

United States Patent Hellner et a1.

[ Dec. 30, 1975 CORROSION-RESISTANT AUSTENITlC-FERRITIC STAINLESS STEEL Inventors: Lars Ivar Hellner, Knektasvagen 31,

S-691 00 Karlskoga; Lars-Ake Norstriim, Villavagen 14, S-683 00 Hagfors, both of Sweden Filed: Aug. 30, 1974 Appl. No.: 502,132

Related US. Application Data Continuation-in-part of Ser. No. 317,494, Dec. 21, 1972, abandoned.

Foreign Application Priority Data Dec. 23, 1971 Sweden 16555/71 US. Cl 148/37; 75/128 C; 75/128 W Int. Cl. C22C 38/02; C22C 38/44 Field of Search 75/128 A, 128 C, 128 N,

References Cited UNITED STATES PATENTS 6/1937 Payson 75/128 W Richardson et a1. 75/128 C de Barbadillo 75/128 W Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or Firm-Elliott l. Pollock preferably and the balance substantially iron and impurities.

2 Claims, N0 Drawings CORROSION-RESISTANT AUSTENITlC-FERRITIC STAINLESS STEEL CROSS-REFERENCE TO RELATED APPLICATION 5 This application is a continuation-in-part of applicants prior copending US. application Ser. No. 317,494 filed Dec. 21, 1972, for Corrosion-Resisting Austenitic-Ferritic Stainless Steel with High Strength now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a corrosion-resisting austenitic-ferritic stainless steel with high strength and THE PRESENT INVENTION The main objective of the present invention is to provide a steel with 1. High general corrosion resistance in salt water,

chloride solutions and others, 2. High resistance to stress corrosion in chloride solutions,

3. High resistance to intergranular corrosion in warm chloride solutions, 1

4. High tensile strength, 5. High ductility and impact strength. A steel with this specific combination of properties has long been sought for-use in centrifugal separators. Prior good ductility and impact strength. The steel according to the present invention, no such steel was availto the invention is intended for use when there are able. The steel according to the present invention severe requirements for corrosion-resistance and thus represents a major improvement for this spestrength. cific application, as well as in other applications Austenitic-ferritic stainless steels with a high chrowhere high tensile strength, ductility and impact mium content are particularly characterized by very strength in combination with corrosion resistance good corrosion resistance even in aggressive environare required. ments. Steels of the Swedish standard type SIS 2324 According to the present invention, it has now, quite (26 Cr, 5 Ni, 1.5 M0) for instance, have many uses surprisingly, been established that a corrosion-resisting where a very high degree of corrosion resistance is steel with considerably higher yield point and ultimate required. This type of steel has rather high strength, but tensile strength as well as good ductility and impact there is a demand for a corrosionresisting steel of this strength, for use in the applications discussed above, type with still higher strength and good ductility and can be achieved by adding a comparatively large quanimpact strength. tity (24% by weight) of Si to a high chromium auste- Common methods of increasing the strength of steel nitic-ferritic steel. are: 30 A steel according to the invention is therefore mainly martensitic hardening characterized by the following composition (percent by precipitation hardening weight):

strain hardening (cold-working) solution treatment C max (H5 Martensitic hardening cannot be used for this type of Si 220 4 0 p y e 35 steel (SIS 2324), since the ferritic matrix cannot be 30 transformed into martensite, and the relatively small Cr 23 27 24 26 portion of the austenite phase present is comparatively {s g8 'g stable (metastable). The possibilities of applying precipitation hardening on ferritic and austenitic-ferritic 40 N max steels with a higher chromium content and thereby increasing the strength by adding some alloying eleand the balance substantially iron and the impurities ments, and by solution treatment and ageing, are very common f hi type f Steel, limited- The g g necessary for precipitation harden It is desirable that the contents of N and C are kept as ing must take place at temperatures at which steels of l as ibl d a lower li i f approx ()1 i this type are embrittled and/or sensitized, i.e. sensitive ibl i i e to intercrystalline corrosion. These types of steel are fd to improve h resistance f h steel to therefore instead used in quench'anneald condition intercrystalline corrosion, the steel can contain such (from PP strong carbide forming elements as Ti, Nb and Ta.

The Possibility of obtaining higher Strength y cold These elements should be included at least in a stoiworking is also limited, as the matrix in these steels is hi i quantity i i i to h content fC N, ferritic to a major extent, which means that the strain P d S present i ll contents t approx, hardening effect will be rather insignificant. Q05 as i i i Solution treatment with Substitutional y g The invention will be described in more detail with ments normally gives little increase 0f the strength. The reference to the teels of which examples are given in normal increase of the yield point of ferrite that can be h follo i T bl expected y a Solution treatment is pp The composition of the steels examined are given in kp/mm for each percent by weight of alloying ele- T bl 1 ments.

Table 1i (Composition in per cent by weight) Steel No. C Si Mn P S Cr Ni Mo N ment of the steel. This is inter alia due to. the fact that Table i s-continued (Composition in per cent by weight) Mn P S Cr Steel No. C Si Ni Mo As will be noted from the table, steels l and 2 have Si contents which exceed those normally used (0.5 l

according to the invention, while steels 7 and 8 have a normal Si content'and' areincluded only for the sake of comparison. Steel 8 is a defined Swedish standard steel u'me austenite in the structure. On the other hand, considering the strength, the austenite content should not exceed 35 by volume. The austenite content should appropriately be kept between 10 and 25 by volume. In order to obtain the desired austenite-ferrite 'proportion inthe steel, consideration must be taken to the The mechanical properties and the austenite con-. tents of the steels indicated in Table l were examined in a quench-annealed condition, and the results are given in Table 2, above. Table 2 clearly proves that Si has a striking strength-increasing effect, and particularly' steels 4,5 and 6 with more 'than 2.5 Si have both Q'0.02 and B far above those of the standard brand. steel (steel 8). ln spite of this', the elongation, the re duction of area and the impact strength of this steel are still favourable. An extremely attractive feature of this hardening effect of Si is also that it is obtained already in the normal quenchannealed condition. The hardening effect given by Si is therefore to be regarded as an extremely strong solution hardening.

The above tables do not indicate any upper limit of 45 Si. Higher Si contents have been tested, and it has been found that increase in,the quantity of this element then gives still higher hardness values. However, higher Si contents than those used do not appear to be realistic.-

At Si contents 2 4%,,hot-working might become diffi- 1 S g' 4 0 cult. This seems to be the. case particularly in combina Mn 0:5 310 tion with a high Mn content a 3%). With an increasg! 3 ing shear resistance of the ferrite, i.e., an increasing 1 hardening effect, there is a greater risk of embrittle- 55 N max 01 a sufficiently small grain size cannot be obtained.

All of the steels as exemplified contain austenite after quench-annealed. The austenite phase is softer than the ferrite phase, which means that particularly the proof This further illustrates the strong hardening effect obtained with a high Si content. I

For ductility and impact strength reasons, however, it is desirable that there be at least approx. by vol- (SlS 2324). Y ferrite and austenite-stabilizing effect to the elements Table 2 Mechanical properties at room temperature and austenite content (per cent by volume) of the steels indicated in Table l in quench-annealed condition. Steel 0.2% Ultimate Elongation Reduction Impact Hardness Austenite Heat-treatment No. proof stress tensile -85 of .area 'strength 1 t HV content 00.2 kp/mm strength 41% KCU kpm/cm per cent 08 kp/mm by Volume 1' 54.1 69.1 26.0 59.7 10.0 244 21 975C,lh,water .2 58.0 69.4 22.0 57.8 11.4 245 13 3- 1 62.8 74.5 23.5 53.2 8.9 a 265 10 ;4 71.1 83.1 21.7 52.7 7.8 278 11 1025C,1h,water 5 69.8 79.3 22.5 57.2 8.5 280 10 975C,1h,water 6 72.1 85.6 1 21.7 51.8 8.0 291 14 1025C,1h,water 7 59.7 67.7 22.3 51.7 9.1 227 5 I 8' 51.0 64.3 27.0 60.4 9.7 230 26 975C,lh,water included. This can be done approximately by means of a socalled Schaeffler diagram.

The invention is not limited to the steels as exemplified above, but the properties can of course be modifled by adding other conventional alloying elements while maintaining the basic composition indicated and the desired: balancebetween austenite and ferritestabilizing elements.

We claim:

'l. Corrosion-resisting austeniticferritic stainless steel having an austenite content of between 10% and by volume and a ferrite content of between 90% and 7 5% byvolume, said steel having high tensile strength, high ductility and impact strength, high general corrosion resistance, high resistance to stress corrosion in chloride solutions, and high resistance to intergranular corrosion in warm chloride solutions, said steel consisting essentially'of (percent by weight): I

- and the balance substantially iron.

2. Steel according to claim 1, consisting essentially of percent by weight): I

stress and the hardness decreases somewhat with in- 60 C max 0 l 5 creasing austenite content. In spite of this steel 7, which s I has the lowest austenite content but only 0.62 Si, has n 2.5

, Cr .24 26 va much lower strength than e.g. steel 6 which has 3.2 I M m Si, but with a considerably higher austenite content. M0 1.2 1.8

' I N max 0.1

andthebalance substantially iron. 

1. CORROSION-RESISTING AUSTENITICFERRITIC STAINLESS STEEL HAVING AN AUSTENITE CONTENT OF BETWEEN 10% AND 25% BY VOLUME AND A FERRITE CONTENT OF BETWEEN 90% AND 75% BY VOLUME, SAID STEEL HAVING HIGH STRENGTH, HIGH DUCTILITY AND IMPACT STRENGTH, HIGH GENERAL CORROSION RESISTANCE,HIGH RESISTANCE TO STRESS CORROSION IN CHLORIDE SOLUTIONS, AND HIGH RESISTANCE TO INTERGRANULAR CORROSION IN WARM CHLORIDE SOLUTIONS, SAID STEEL CONSISTING ESSENTIALLY OF (PERCENT BY WEIGHT): C MAX 0.15 SI 2.0 - 4.0 MN 0.5 - 3.0 CR 23 - 27 NI 3.5 - 8.0 MO 0.5 - 2.0 N MAX 0.1 AND THE BALANCE SUBSTANTIALLY IRON.
 2. Steel according to claim 1, consisting essentially of (percent by weight): 